Argument mapping

Argument mapping is a set of techniques and notations for representing the structure of arguments. The idea is similar to that of diagramming sentences.

Mapping an argument allows one to see which arguments have been presented to support or disprove an assertion. There are also techniques for highlighting poorly-supported assertions, unsound arguments, hidden premises, and so forth.

An argument diagram is typically represented as a tree of assertions: at the root of the tree is the main issue under discussion, e.g., "God exists" or "the death penalty should be abolished". Underneath that are boxes representing arguments for or against the main assertion, connected to it with different types of lines or arrows to indicate whether they support the main contention or not. These arguments, in turn, can have other arguments for or against them.

Bookkeeping rules

According to argument mapping rules, each contention should be supported not by a single premise, but by two or more co-premises, so that the co-premises and the contention together form a syllogism. For instance:

Contention: government subsidies of tobacco farmers should cease.

Premise: tobacco subsidies cause harm to the public.

This example violates the rule that there should be at least two co-premises for each contention. Furthermore, "Tobacco subsidies cause harm to the public, therefore the government should cease subsidising tobacco farmers" is not a proper syllogism. We can fix this by adding a second co-premise, as follows:

Contention: government subsidies of tobacco farmers should cease.

Premise: tobacco subsidies cause harm to the public.

Premise: the government should minimize harm to the public.

Another rule is that, much as in double-entry bookkeeping, every significant term or phrase should appear in two places, either in the contention or in the premises. This helps ensure that important terms are not lost, and do not appear out of nowhere. For example, consider:

Contention: Socrates is mortal.

Premise: Socrates is a man.

We already know that this example is flawed because there should be at least two premises. To figure out what the missing co-premise should be, we note that the terms "man" and "mortal" only appear once: one in the contention, and one in the premise. We can add a second co-premise that includes these terms: